CAMROSE, Alberta - Organizers on Sunday canceled the closing day of a country music festival in central Alberta a day after a fierce thunderstorm caused an outdoor stage to collapse, killing one person and injuring about 75 others.

Thousands of fans were camped out at the Big Valley Jamboree in Camrose, 60 miles (100 kilometers) southeast of Edmonton, when strong winds and heavy rain struck about 6 p.m. on Saturday, sending people screaming and running for cover. Actor Kevin Costner who was about to perform with his band narrowly escaped serious injury.

The event is billed as Canada's largest country music festival with attendance estimated at 15,000.

Police and festival organizers told a news conference at the site Sunday that one person was killed by falling debris from the stage and about 75 others were injured, most of whom were treated at the site.

Camrose Police Chief Darrell Kambeitz said about 21 of the injured were taken to hospitals, two of whom were in critical condition.

At the news conference, officials made it clear that had very little advance warning of the severe storm. Kambeitz said the Royal Canadian Mounted Police informed local police at 5:55 p.m. about unconfirmed reports that a possible tornado had touched just south of Edmonton.

"We had people on the stage at 5:57 p.m., and the storm struck between 5:57 p.m. and 6 p.m," he said.

Jamboree producer Larry Werner said his office also got a call saying a severe windstorm was heading directly for the venue and immediately let the performers know they had to get off the stage so an announcement could be made for the crowd to clear the concert bowl.

But the warning came too late. Fearing a tornado, panicked fans scrambled to find loved ones and shelter after the storm struck.

"We were all racing for the exit," said Lori Trelenberg of Sherwood Park, Alberta. "It was devastation. It was strong and powerful. The stage just sort of crumbled."

The Nashville-based Billy Currington band was playing when the power went out and the stage collapsed. One band member was pulled from the wreckage with a broken arm.

Costner and his band Modern West were getting ready to perform next when the stage caved in. Costner's manager, Nick Meinama, said both he and the actor were caught underneath the stage.

"He was unhurt. I was unhurt. We wiggled our way out," Meinama told CTV News Sunday. "We found that our road manager and guitar player had been hurt, but the ambulance and the paramedics were here instantly to take them to the hospital."

Meinama said the two were later released from the hospital and were flying back to the U.S.

CFCW radio personality Danny Hooper was on the stage when the storm hit.

"I can't describe the sky _ it was brown and purple and green," Hooper said on CFCW. "The massive wind blew me backwards."

Claire Beaudoin was sitting only three rows from the stage when the storm struck, suddenly sending musical instruments, chairs and other debris flying. Beaudoin saw people falling.

"What went through my mind was `Oh, my God No!' It was really scary. People were crying," she said.

Vancouver-based country music singer Jessie Farrell, who had performed earlier Saturday, said it was a terrifying experience.

"It felt like bombs were going off around us in this concrete and steel building," she told CTV News. "Huge hits of power hitting the building, and then the lights were off."

Officials could not say immediately why the structure collapsed, or how long it would take to get answers. Provincial officials are helping with the investigation.

Heavy rain deluged the area for about an hour as emergency crews treated the injured and combed through the wreckage for more victims. The site was cordoned off by police as searchers worked through the night under blazing spotlights hung from massive cranes. The cranes were still clearing debris from the site Sunday.

The four-day festival in central Canada started Thursday and had been scheduled to wrap up Sunday.

Sunday's program was cancelled by the promoter, who said the organizers were deeply saddened by the tragedy.

"Our thoughts and prayers go out to all of the families, fans and friends impacted by this incident," producer Werner said in a statement.

Social networking sites were quickly abuzz with reaction.

On Twitter, the Oak Ridge Boys sent this message: "Our prayers are with the Big Valley Jamboree. We have been there six times, including last summer. We know these folks."

Environment Canada had issued a thunderstorm watch for the area Saturday and a similar watch remained in effect across much of central Alberta on Sunday. Forecasters noted that a line of thunderstorms marching across the region had the potential to produce high winds, hail, local downpours and lightning.

Saturday's severe storm system was also blamed for the death of a toddler in Calgary who was killed when a piece of metal blew off a building under construction. Two other people on the sidewalk below were injured.